Typically, fine irregularities are formed on a surface of a solar cell to suppress surface reflection. The incident light is subjected to a multiple reflection by the fine irregularities and is efficiently absorbed into the inside of the solar cell. These fine irregularities are called a texture.
Generally, in a monocrystalline-silicon solar cell, a texture is formed by wet etching using an alkaline aqueous solution such as NaOH or KOH and isopropylalcohol (IPA). Because this technique uses a difference of etching speeds on a crystal plane, it is effective when a wafer is configured by a single crystal plane like a monocrystalline silicon; however, the reflectance cannot be sufficiently decreased when various crystal planes are present within a plane like a polycrystalline silicon.
Therefore, a texture forming method that is not dependent on a crystal plane orientation, such as a mechanical processing method (for example, Patent Document 1) and a reactive ion-etching method (for example, Patent Document 2), has been studied. The mechanical processing method is a single-wafer process, and the reactive ion-etching method uses a vacuum equipment although a certain number of wafers can be batch processed. Consequently, these methods take considerable processing costs.
Therefore, the following method of forming a texture is disclosed (Patent Document 3), in which a porous silicon layer is formed on a surface of a silicon substrate by dipping the silicon substrate into a mixed aqueous solution of oxidizing reagent containing a metal ion and hydrofluoric acid. Thereafter, the substrate formed with the porous silicon layer is dipped into alkali, thereby forming a texture. In relation to this, a mechanism of forming a pit on silicon adhered with a metal is described in Patent Document 4.    Patent Document 1: Japanese Patent No. 3189201
Patent Document 2: Japanese Patent Application Laid-open No. H09-102625    Patent Document 3: Japanese Patent No. 3925867    Patent Document 4: Japanese Patent Application Laid-open No. 2004-71626